THE CENTRAL NERVOUS SYSTEM. 



113 



system is the most suitable to commence with. It is formed 

 from the epiblast, which is the earliest of the germinal layers to 

 be definitely established ; it appears at a very early stage ; and 

 it plays a prominent part, especially in the younger embryos, in 

 determining the shape and proportions of the body. 



1 . General History of the Central Nervous System. 



The epiblast of the frog, as already described, consists, 

 almost from the first, of two layers, the distinction between 

 which is established before the close of the period of segmenta- 

 tion. Of these, the upper or epidermic layer is a single stratum 

 of closely fitted, short columnar or cubical cells ; while the lower 

 or nervous layer (Figs. 51, 52) consists of spherical or ovoid 

 cells, more loosely arranged, and two or three deep : it is from 

 this lower layer that the nervous system is developed. 



The first trace of the nervous system appears at a stage 

 immediately succeeding that shown in Fig. 55, when the 

 embryo is ellipsoidal in shape, and the blastopore has become 

 much reduced, and less conspicuous owing to the yolk-plug 

 having withdrawn from 

 the surface. 



The dorsal surface of 

 the embryo now flattens 

 slightly, and along the 

 flattened area the deeper 

 or nervous layer of the 

 epiblast thickens to form 

 the neural plate, a tri- 

 angular area extending 

 aJong the back of the 

 embryo, wider in front 

 but narrowing poste- 

 riorly towards the blas- 

 topore. Slightly raised 

 ridges, the neural folds 

 (Fig. 57, NF), soon ap- 

 pear, bordering the neural 

 plate laterally ; and a shallow neural groove (Figs. 56, 57, NG) is 

 formed along its dorsal surface in the median line, extending 

 forwards from the blastopore. 



i 



Fid. 57. A Frog Embryo at the time of ap- 

 pearance of the neural folds : seen from 

 the dorsal surface, x 20. 



N"P, neural fold : the reference line points 

 to the junction of the anterior and the left 

 lateral folds. NG, neural groove. YP, yolk 

 plug, greatly reduced in size, but still visible 

 through the blastopore. 



